President Obama on Monday bluntly accused pro-Moscow separatist fighters in Ukraine of stealing evidence and improperly removing bodies from the crash site of Malaysia Airlines Flight 17 and pressed Russia to compel the rebels to stop blocking an international investigation.

“What exactly are they trying to hide?” Obama asked in a hastily arranged statement on the south lawn of the White House. “The burden now is on Russia to insist that the separatists stop tampering with the evidence.”

The president noted that it has been four days since the passenger jet went down, killing all 298 people aboard. The United States blames a surface-to-air missile fired from territory controlled by the rebels, who have extensive support from Moscow, including advanced weapons and training.

International investigators — including an American team — have traveled to Ukraine to carry out a full investigation.

“Unfortunately, the Russian-backed separatists who control the area continue to block the investigation. They’ve repeatedly prevented international investigators from gaining full access to the wreckage,” including by firing their weapons into the air when the teams approached the site, Obama charged.

“These separatists are removing evidence from the crash site,” the president said. “Moreover, these Russian-backed separatists are removing bodies from the crash site, oftentimes without the care that we would normally expect.”

“Russia has extraordinary influence over these separatists — no one denies that,” Obama underlined. “Russia has urged them on. Russia has trained them. We know that Russia has armed them with military equipment and weapons including anti-aircraft weapons.”

“Given its direct influence over the separatists, Russia — and President Putin in particular — has a direct responsibility to compel them to cooperate with the investigation,” Obama said.